1. Field of the Invention
The present invention relates to a reflective-type liquid crystal display device which can display information on both surfaces.
2. Description of the Related Art
The characteristics of a conventional liquid crystal display device has been low power consumption, low profile and light weight, but in pursuing visibility and color reproducibility, it developed into a transmission-type liquid crystal display device provided with a back light, in which the original characteristics of the liquid crystal display device are not shown enough. However, with the rapid diffusion of a personal digital assistant recently, a reflective-type liquid crystal display device employed thereto is rapidly improving.
The most remarkable characteristic of the reflective-type liquid crystal display device is the low power consumption. Since most personal digital assistants are operated by batteries, the life of the battery influences the performance of the digital assistant. In the digital assistant like this, it is suitable to employ the liquid crystal display device, and particularly, the reflective-type liquid crystal display device which can reduce the power consumption to prolong the battery""s life for several hours, since it does not use the back light.
Here, operations of a conventional liquid crystal display device will be briefly described with reference to FIG. 9.
A conventional reflective-type liquid crystal display device 100 shown in FIG. 9 is structured by disposing a reflector 102 on one surface side of a liquid crystal panel 101 (on the lower surface side in the drawing).
The liquid crystal panel 101 is structured by holding a liquid crystal layer between two transparent substrates with electrodes formed thereon, wherein a TN liquid crystal (twisted nematic liquid crystal) at a twist angle of 90 degrees or an STN liquid crystal (super twisted nematic liquid crystal) at a twist angle of 180 degrees to 270 degrees are often used as the liquid crystal layer. Polarizing films (not shown) are adhered to both sides of this liquid crystal panel 101. Each of the polarizing films is disposed so that the directions of these transmission axes correspond respectively with the alignment directions of the upper surface side and the lower surface side of the liquid crystal layer.
The reflector 102 has a structure in which a coating made of aluminum or silver is formed on a film and has a high reflectance.
Supposing that the liquid crystal panel 101 of this reflective-type liquid crystal display device 100 is disposed on the visible side. In this case, an incident light a is made incident from the visible side of the liquid crystal panel 101 to pass through the liquid crystal panel 101, and then reflected by the reflector 102 to become a reflection light b, returned to the visible side and emitted out. The liquid crystal panel 101 can display characters and the like by the contrast between reflection and non-reflection.
However, since the reflector 102 is disposed on one side of the liquid crystal panel 101 in the conventional reflective-type liquid crystal display device 100, the excellent display can be obtained when the liquid crystal panel 101 is disposed on the visible side, but when the reflector 102 is disposed on the visible side, it is impossible to view its display since the incident light is interrupted by the reflector 102 and cannot reach the liquid crystal panel 101.
This is the matter of course in the conventional reflective-type liquid crystal display device since the reflector 102 is provided on one surface side of the liquid crystal panel as described above, which has never been discussed before.
However, the reflective-type liquid crystal display device is advantageous in low power consumption, small size, light weight and low profile, which is most suitable for the display device of the personal digital assistant, but it is preferable to display information on both surfaces like a paper as being a candidate for the replacement of the paper in the age of paperless. Thus, the amount of displayed information per paper doubles, as well as the practical applications increase, whereby the liquid crystal display device becomes to have various uses.
As a means for displaying information on both surfaces in the conventional liquid crystal display device, it is possible to think of attaching two reflective-type liquid crystal display devices to each other. However, there arises problems that the cost is increased as two liquid crystal display devices are needed, its advantages of low-profile and light-weight are eroded, and power consumption doubles, with the result that the liquid crystal display device in the above design can not put into practical use.
The present invention is made to solve the aforementioned problems in a liquid crystal display device, and its object is to make it possible to display information on both surfaces of the liquid crystal display device, which can be viewed from both sides, and to satisfy the needs of low cost, low profile, light weight and low power consumption.
The liquid crystal display device of the present invention is comprised of a liquid crystal cell formed by sealing a liquid crystal layer between a pair of transparent substrates having electrodes respectively on opposed inner surfaces thereof, reflection-type polarizing films respectively disposed on both sides of the liquid crystal cell, for transmitting a linearly polarized light in a direction parallel to a transmission axis and reflecting a linearly polarized light in a direction orthogonal to the transmission axis, and absorption-type polarizing films respectively disposed outside the reflection-type polarizing films, for transmitting a linearly polarized light in a direction parallel to a transmission axis and absorbing a linearly polarized light in a direction orthogonal to the transmission axis.
It is preferable that the reflection-type polarizing film and the absorption-type polarizing film disposed on the same side of the liquid crystal cell are disposed so that the directions of the transmission axes thereof almost correspond with each other.
Further, it is preferable that a light scattering member is disposed at least one place between the liquid crystal cell and the reflection-type polarizing film on one surface side and between the liquid crystal cell and the reflection-type polarizing film on the other side.
Moreover, a super twisted nematic liquid crystal can be used as the liquid crystal layer of the liquid crystal cell. In this case, it is preferable that a retardation film is disposed on one surface side of the liquid crystal cell.
Furthermore, a twisted nematic liquid crystal can be used as the liquid crystal layer of the liquid crystal cell. Also, a ferroelectric liquid crystal can be used as the liquid crystal layer of the liquid crystal cell.